Signed by Colin Todd, Hewlett joined from local rivals Bristol City on a Bosman free transfer during the 2000/01 pre-season, making his debut on the opening day of the season in a 0-0 draw with Colchester. His first season was somewhat spoiled by injury, but his return to the side brought an upturn in results, ultimately helping the Town keep their Second Division status.

Over the 2001/02 season, Hewlett became a bit of a scapegoat for the fans - for a supposed ball-winning, protecting midfielder, at times he seemed unwilling to get stuck in. Despite this, he consistently remained in both Roy Evans' and Andy King's teams - and after sorting out a short dispute over it's length, he signed a two year extension to his contract at the end of the season.

After missing just six games during 2002/03, Hewlett again featured the following season - until he was dropped for a match at Brentford in December 2003, which the Town won 2-0. Since establishing himself in the Swindon side, it was the first game that Hewlett had missed that wasn't enforced by injury - and it had the desired effect, with Hewlett putting in some much improved performances after his return to the side. When regular captain Andy Gurney was suspended in January, Hewlett was handed the captaincy - and the Town went on on a run of eleven games without defeat, with Hewlett keeping the armband despite Gurney's return. He remained as captain for the remainder of the campaign, and picked up the Supporter's Trust "Most Improved Player" award at the end of the season.

An alleged summer spat with Andy Gurney was perhaps the catalyst for a drop in Hewlett's performances at the start of the 2004/05 season, and though Gurney left the club, Hewlett still found himself on the end of the same criticisms he had been receiving in 2003 and before. Nevertheless, he still commanded a regular place in Andy King's line-up, and he was generally seen as one of King's favourites. It therefore came as somewhat of a surprise in April, when despite being fit, Hewlett wasn't even named in the squad for the match against his former club, Bristol City - and details soon emerged that Hewlett had himself asked not to be involved, in a week where King was deciding which of the out-of-contract players would be offered new deals. Hewlett appeared in the Evening Advertiser, claiming that the offer made to him was unacceptable - the following week, King claimed that he had told Hewlett he would be released, and that no such deal was offered. Having played in exactly 200 games for the Town in all competitions, Hewlett didn't feature in any of the last five games of the season, and he left the club at the end of the season.